#include<bits/stdc++.h>
#define ll long long
#define L x<<1
#define R x<<1|1
#define mid ((l+r)>>1)
#define lc L,l,mid
#define rc R,mid+1,r
#define Root 1,1,n
#define OK Ll<=l&&r<=Rr
#define rep(x,y,z) for(int x=(y);x<=(z);x++)
#define per(x,y,z) for(int x=(y);x>=(z);x--)
#define repn(x) rep(x,1,n)
#define repm(x) rep(x,1,m)
#define pb push_back
#define e(x) for(int i=h[x],y=to[i];i;i=nxt[i],y=to[i])
#define E(x) for(auto y:p[x])
#define Pi pair<int,int>
#define ui unsigned ll
#define YY puts("Yes"),exit(0)
#define NN puts("No"),exit(0)
#define yy return puts("Yes"),void()
#define nn return puts("No"),void()
inline int read(){int s=0,w=1;char c=getchar();while(c<48||c>57) {if(c=='-') w=-1;c=getchar();}while(c>=48&&c<=57)s=(s<<1)+(s<<3)+c-48,c=getchar();return s*w;}
inline void pf(int x){if(x<0) putchar('-'),x=-x;if(x>9)pf(x/10);putchar(x%10+48);}
using namespace std;
const int N=1e6+5,M=1e6+5,inf=(1LL<<30)-1,mod=1e9+7;
const ll llf=1e18;
inline void add(int &a,int b){((a+=b)>=mod) and (a-=mod);}
inline int Add(int a,int b){return add(a,b),a;}
inline int mul(int a,int b){return 1LL*a*b%mod;}
inline void Mul(int &a,int b){a=mul(a,b);}
inline void red(int &a,int b){add(a,mod-b);}
inline int Red(int a,int b){return red(a,b),a;}
inline int qp(int a,ll b){if(!b)return 1;int c=qp(a,b>>1LL);Mul(c,c);if(b&1)Mul(c,a);return c;}
inline int INV(int x){return qp(x,mod-2);}
inline bool che(int x,int p){return (x>>p)&1LL;}
int n,k;
double p[N],pr[N],Pr[N],Sf[N];
inline bool cmp(double a,double b){
	return a>b;
}
int st[N],tp;
inline double solve(int i){
	tp=0;
	int res=k-i;
	pr[0]=1;
	double ans=1;
	rep(j,1,i)pr[j]=Pr[j];
	rep(j,1,res)pr[j+i]=pr[j+i-1]*p[n-res+j];
	ans-=pr[k];
	double S=1.0;
	per(j,res,1)S=Sf[n-res+j],ans-=S*pr[j+i-1];
	per(j,i,1)S*=(1.0-p[j]),ans-=S*pr[j-1];
	return ans;
}
inline void Main(){
	n=read(),k=read();
	repn(i){
		int a=read(),b=read();
		p[i]=(double)a/b;
	}
	sort(p+1,p+n+1,cmp);
	Pr[0]=Sf[n+1]=1;
	repn(i)Pr[i]=Pr[i-1]*p[i];
	per(i,n,1)Sf[i]=Sf[i+1]*(1.0-p[i]);
	int l=1,r=k-1,p=1;
	double ans;
	if(k!=n)while(l<=r){
		double A=solve(mid),B=solve(mid+1);
		if(B<=A)p=mid,l=mid+1;
		else r=mid-1;
	}
	if(k!=1)ans=min(solve(p),solve(p+1));
	else ans=solve(p);
	printf("%.7lf\n",ans);
}
signed main(){
	freopen("sleep.in","r",stdin);
	freopen("sleep.out","w",stdout);
	int T=1;
	while(T--)Main();
	return 0;
}
